1. Field of the Invention
The present invention relates generally to systems and methods for attaching silicon chips to substrates or package bases. More particularly, the present invention involves an improved metallization system for use in such chip attachments.
2. Description of Related Art
The use of eutectic solders, such as gold/tin is a common method for attaching silicon chips to substrates. This technique ensures good heat dissipation and long-term reliability. This attachment procedure involves placing a gold/tin preform between the substrate and a gold layer on the silicon chip and applying heat, to form a gold/tin alloy which functions as a bonding layer between the bottom surface of the silicon chip and the top surface of the substrate. Pure gold melts at temperatures of approximately 1100.degree. C. while pure tin melts at temperatures above 232.degree. C. The 80 percent gold/20 percent tin alloy melts at a temperature of around 280.degree. C. This relatively low melting temperature allows one to attach silicon chips to substrates at temperatures of between 300 to 350.degree. C.
One method for attaching silicon chips to a substrate involves forming a metallization system on the backside of the chip. A metallization system which is presently being used in the industry is shown in FIG. 1. This metallization system includes a layer of chromium and a layer of gold. The layers are typically about 1000 angstroms and 4000 angstroms thick, respectively. The chromium is applied to form an adhesion layer for the gold, while the gold is used primarily to form a mask for the etchants used in the wafer thinning process. During an annealing step at 400.degree. C., the metal layers react and allow a silicon-rich alloy to form on the surface. If oxygen is present in the processing atmosphere, the silicon which reaches the surface is converted into silicon dioxide. The presence of small amounts of silicon dioxide adversely affects chip attachment. As a result, all chip attachment processing utilizing this type of system is typically performed in a vacuum. This technique limits the amount of silicon which oxidizes during any heat treatment and/or during chip attachment.
It would be desirable to provide a metallization system which allows the necessary silicon alloying for chip attachment without the requirement of a vacuum environment. The need for a vacuum environment makes the present chip attachment process time consuming and expensive. Further, it would be desirable to provide a metallization system which could be used in conjunction with a nitrogen belt furnace. Nitrogen belt furnaces are conventional equipment which are designed to be used in processing large numbers of silicon chips.